Conversion of natural gas to liquid hydrocarbons (“Gas To Liquids” or “GTL” process) is based on a 3 step procedure consisting of: 1) synthesis gas production; 2) synthesis gas conversion by FT synthesis; and 3) upgrading of FT products (wax and naphtha/distillates) to final products such as naphtha, kerosene, diesel or other products, for example lube oil base.
Supported cobalt catalysts are the preferred catalysts for the FT synthesis. The most important properties of a cobalt FT catalyst are the activity, the selectivity usually to C5 and heavier products and the resistance towards deactivation. Known catalysts are typically based on titania, silica or alumina supports and various metals and metal oxides have been shown to be useful as promoters.
In a paper by Iglesia et al. [“Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets and Reactors” Advances in Catalysis, 39(1993) 221, a Thieles modulus is defined as a product of two components, Ψn and χ, where Ψn depends only on the diffusivity and reactivity of the individual molecules, whereas χ depends only on the physical properties and site density of the catalyst. They have described a model whereby the selectivity to C5+ products can be described as a volcano plot in terms of χ. The structural parameter is given as:χ=Ro2Φθm/rp,where θm the site density, e.g. as the number of surface atoms of Co metal atoms per cm2 of pore area in the catalyst particle, Ro is the diffusion length, i.e. the radius of an essentially spherical catalyst particle, Φ is the porosity of the particle (cm3 pore volume/cm3 particle volume) and rp is the mean pore radius.
Now, the site density in the above equation can be rewritten as:θm=(WCoD NAρcatrp)/(MwCo2Φ)where WCo, D and MwCo are the weight fraction of Co in the catalyst particle, the dispersion of Co (the number of exposed metal surface atoms to the total number of Co atoms in the particle) and the molecular weight, respectively. NA is Avogadro's number and ρcat the catalyst density (g/cm3). Inserting the latter expression for the site density yields:χ=(Ro2WCoD NAρcat)/(2MwCo).
It is then obvious that χ only depends on a universal constant, characteristic data for cobalt in the catalyst as well as the size and density of the catalyst particles. It is particularly significant that χ does not depend on the pore radius, rp. Now, surprisingly it has been found that the selectivity of the Fischer-Tropsch reaction to C5+ products indeed do depend on the pore size.
In a paper by Saib et al. [“Silica supported cobalt Fischer-Tropsch catalysts: effect of pore diameter of support” Catalyses Today 71(2002) 395-402], the influence of the effect of the average pore diameter of a silica support on the properties of a cobalt catalyst and their performance in F-T synthesis is discussed. The article concludes that the support pore diameter has a strong effect on cobalt crystallite size with larger crystallites forming in larger pore sizes. Also, the activity was found to be a function of the metal dispersion and the maximum C5+ selectivity a function of the conversion.
In EP 1 129 776 A1 it is argued that internal diffusion phenomena in a catalyst particle depend on the chemical and morphological structure of the catalyst (pore dimensions, surface area, density of the active sites) and on the molecular dimensions of the species in question. This is a general teaching found in relevant textbooks, e.g. expressed in terms of the Thiele modulus, and it is significant that the pore dimension, i.e. the pore radius or diameter is one of the critical parameters. Further, it is taught that for the Fischer-Tropsch synthesis, interparticle diffusion will create low concentrations of CO towards the centre of the particle with a consequent progressive rise in the H2/CO ratio inside the catalyst and that this condition favours the formation of light hydrocarbons (lower α-value and C5+ fraction). On the other hand, it is stated that multiphase reactors of the slurry type generally use small catalyst particles (20-150 μm) which do not give internal diffusion problems, and more specifically that for catalysts based on differently supported cobalt used in the Fischer-Tropsch synthesis, it is possible to neglect internal diffusion limitations by operation with particles having diameter of less than 200 μm. Reference is made to Iglesia et al., Computer-aided design of catalysts, E D. Becker-Pereira, 1993, chap. 7. This patent claims the benefit of particles in the range 70-250 μm to simplify the liquid/solid separation step in the process, while not negatively influencing the effectiveness of the catalyst.
To summarise, in EP 1 129 776 A1 and references therein, it is taught that regardless of pore dimension, the selectivity of the catalyst will not be affected as long as the catalyst particle diameter is below 250 μm, or at least below 200 μm. Now, we have very surprisingly found that even for small particles with an average size between 50-80 μm, the selectivity does vary with the pore size, specifically, larger pores give higher C5+ selectivities.
In EP 0 736 326 B1, it is shown that the C5+ selectivity can increase over a certain range of increasing pore size for a cobalt on alumina type FT catalyst. However, no reference or details of the method of measuring pore size is given, and it is well known that reported values vary significantly with method, e.g. for different probe gases or whether adsorption or desorption isotherms are employed. The pore size was essentially increased by using high calcination temperatures, a procedure that may adversely affect the attrition resistance of the catalyst. Comparably moderate catalyst pore volumes were also used, thus giving more dense particles that may be less favourable in a slurry reactor environment. No effect on selectivity with varying pore volumes was reported. Unfortunately, the reported particle sizes used in the tests are inconsistent and can therefore not be considered, more so as the low selectivity (and smallest pore size) data seem to be based on extruded catalyst samples. It is well known that large particles, typical of extrudates or coarse fractions thereof, will give low C5+ (or liquid) selectivities due to diffusion limitations giving an efficient enhanced H2/CO ratio inside the particles. This results in some very low liquid selectivities reported in EP 0 736 326 B1, in the range 40 to 65 wt %. Above 65 wt % liquid, there is no reported influence of pore size or pore volume in EP 0 736 326 B1.